Created concrete material by following a 2.79 tutorial in 2.8 and messed up true displacement
Due to adding vector displacement not so long ago to Blender you have to use dedicated displacement node to convert scalar to vector displacement.



I would say that your shader structure is outdated. It is better to mix RGBs instead of shaders, and use Principled BSDF instead of mixing Diffuse and Glossy. Take a look at the blend file. Although this is not the best example to demonstrate the superiority of the approach, I changed it so it would reflect the idea without altering the artistic component. The shader became heavier but more realistic and PBR.